[test] Cover `'use cache: private'` in route handlers (#97349)
The `use-cache-private` suite covered private caches in server
components only, so this adds a route handler that calls one `'use
cache: private'` function three times: a concurrent pair, and a third
call after a short delay. The concurrent pair joins a single in-flight
invocation, while the delayed call lands after the map of pending
intra-request invocations has dropped its entry, so it is served from
the completed entry that the request retains. Each call passes a fresh
object literal, which makes the React `cache()` memo miss on reference
equality so that the lookup falls through to the serialized cache key.
The cache function also reads a cookie, which covers that request APIs
are allowed inside a private cache in a route handler.
A second test asserts that nothing is shared across requests. In
production private entries are neither persisted nor deduped across
requests, so every request re-runs the cache function. In development
they are persisted and forced to `revalidate: 0`, so the next request is
served the stale entry while a fresh one warms in the background, which
the dev branch of the test follows. Both tests pass their own `id`
search param, and therefore use their own cache key, so the entry that
dev persists for one is never served to the other.
This also drops the claim that the directive is not available in route
handlers from the `'use cache: private'` reference. Nothing in the
source rejects it, and it was already allowed for request work unit
stores when that note was written.
